By means of such a lance, a liquid fuel, for example a suitable oil, and a gaseous fuel, for example natural gas, can be sprayed alternatively or in a cumulative manner into a hybrid burner. The lance is normally supplied with the gaseous fuel via a pipeline in which a gas pressure predetermined by the gas supply system prevails. However, in a multiplicity of applications, e.g. in a combustor having a low-pressure burner and downstream high-pressure burner, this system pressure present in the pipeline is too low in order to be able to spray the gaseous fuel with sufficient pressure difference through the lance into the combustor. Accordingly, it is conventional practice to arrange an additional compressor upstream of the lance in order to raise the gaseous fuel to the requisite pressure level. However, the fitting of such an additional compressor increases the installation costs of the combustor or of the gas turbine equipped with it. Furthermore, the additional compressor, for its operation, requires energy which, in a preferred application of the gas turbine in a power plant for the generation of electricity, reduces the efficiency of the power plant.